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MDS-D/DH Series Specifications Manual
7-1 Selection of the servomotor
7-1-2 Selection of servomotor capacity
The following three elements are used to determine the servomotor capacity.
1. Load inertia ratio
2. Short time characteristics (acceleration/deceleration torque)
3. Continuous characteristics (continuous effective load torque)
Carry out appropriate measures, such as increasing the motor capacity, if any of the above conditions is not
fulfilled.
(1) Load inertia ratio
Each servomotor has an appropriate load inertia ratio (load inertia/motor inertia). The control becomes
unstable when the load inertia ratio is too large, and the servo parameter adjustment becomes difficult. It
becomes difficult to improve the surface precision in the feed axis, and the positioning time cannot be
shortened in the positioning axis because the settling time is longer.
If the load inertia ratio exceeds the recommended value in the servo specifications list, increase the motor
capacity, and select so that the load inertia ratio is within the recommended range.
Note that the recommended value for the load inertia ratio is strictly one guideline. This does not mean that
controlling of the load with inertia exceeding the recommended value is impossible.
1. When selecting feed axis servomotors for NC unit machine tools, place importance on the surface
precision during machining. To do this, always select a servomotor with a load inertia ratio within
the recommended value. Select the lowest value possible within that range.
2. The load inertia ratio for the motor with brakes must be judged based on the motor inertia for the
motor without brakes.
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